Molybdenum test limiter experiments in TEXTOR

Abstract

Limiter experiments with a MO test limiter have been carried out in TEXTOR under various conditions with ohmic and NBI heating. Maximum power loads reached about 20 MW/m2 resulting in surface temperatures up to the melting point. A maximum fraction of 8% of the total convective energy in the plasma was deposited onto the MO limiter. Molybdenum impurities are mainly produced by physical sputtering due to the impact of C and 0 impurities. Under ohmic heating conditions the MO impurity radiation increased with increasing plasma density, though the relative source strength of the MO release decreased. Approaching a critical density of about 3 X lOI3 cm-3 MO accumulated in the plasma centre leading to strong central radiation with hollow temperature profiles, which resulted in a minor disruption. Under NBI heated plasma conditions (CO-NBI, 1.3 MW) MO radiation decreased with increasing plasma density as did the relative MO source at the limiter; at high plasma densities the influence of the MO limiter on plasma impurities and plasma performance became negligible. No instability was observed under these conditions. The different behaviour between ohmic and NBI heating conditions is explained by the different relative importance of neoclassical and anomalous transport under ohmic and NBI heating, respectively. The observed instability in high density ohmic plasmas is caused by MO accumulation in the plasma centre and is attributed to neoclassical transport processes.

Molybdenum sources and transport in the Alcator C-Mod tokamak

“…Source rate calculations have also been routinely made, especially in tokamaks with low-Z plasma interaction surfaces [5,6]. However, calculations involving high-Z plasma interaction surfaces have become more frequent the last few years [7][8][9].…”

High-Z plasma facing components in fusion devices: boundary conditions and operational experiences

“…Tungsten and molybdenum were introduced into TEXTOR as a dome shaped testlimiter through a dedicated limiter lock [88,89] or as a poloidal main limiter on a retractable mounting (10 W-coated tiles covering about 20% of the poloidal circumference) [90]. Most of the data were obtained with the test limiter 0.5 cm inside the radius defined by the main toroidal limiter and with 1.5 MW of neutral beam heating, resulting in power flux densities between about 8 and 12 MW m £ 2 [91].…”

“…Most of the data were obtained with the test limiter 0.5 cm inside the radius defined by the main toroidal limiter and with 1.5 MW of neutral beam heating, resulting in power flux densities between about 8 and 12 MW m £ 2 [91]. The accumulation of high-Z impurities occurred very reproducibly under pure ohmic heating conditions [88] when a critical plasma density was reached (see Fig. 8), although the W and Mo limiters were loaded with only a fraction of about 4-6% of the total convective power.…”

Analysis of fuel retention in plasma-facing components from controlled fusion devices

“…In particular, metal walls are only physically sputtered by the plasma with low sputtering yield if incident ion energies are kept below a threshold value, and thus avoiding the contribution from chemical erosion specific to carbon. For instance, tungsten-coated graphite tiles have been tested in ASDEX-U on the main vessel wall and in the divertor [27][28][29], bulk tungsten test limiters in TEXTOR [30][31][32][33][34][35] and bulk molybdenum in Alcator CMod [36] and TEXTOR (test limiters) [37]. Beryllium (Be) has been used regularly in JET: evaporated Be layers on the main chamber wall and solid Be components (Faraday screens) of antennas for ion cyclotron resonance heating [38].…”